Fractionator

ABSTRACT

A fractionator includes a shaft attached to a drum which is positioned in a housing and which is rotatable about a substantially horizontal axis and has an inlet means for suspension and at least two outlets for fractions. The drum has a first end and a second end and includes at least two fluid tight walls surrounding the axis. The drum also extends in its longitudinal direction between the first end and the second end to define a fluid channel. The fluid channel extrends substantially parallel to the axis from the first end to the second end and continuously in a circumferential direction of at least 360°.

The present invention concerns a fractionator for fractioning asuspension in at least two fractions, including a drum rotatable about asubstantially horizontal shaft and having an inlet for suspension and atleast one outlet for fractions, said drum including at least two wallscurved about the shaft and extending substantially parallel thereto inthe longitudinal direction thereof, said walls defining between them afluid channel substantially parallel to the shaft.

Within the pulp industry there is a need to separate from certainfiltrates particles accompanying the filtrate, for instance in themanufacture of recycled paper pulp. In one phase of this process, therecycled pulp is washed, which results in that particularly fillers,such as clay and ash, printing ink and so-called fines are washed out ofthe pulp in a filtering process, which aims at retaining mainly longfibres. However, it is unavoidable that also long fibres accompany thefiltrate, which, thus, will contain long fibres, fines, fillers andprinting ink. It is, of course, desirable to be able to recover not onlythe valuable long fibres, but also fines and fillers, of whichparticularly the latter are valuable and are needed in papermaking.Besides, for environmental reasons it is favourable if also the finestparticles can be recovered and recycled.

In order to separate particles of different sizes in a liquid, it isknown to utilize a so-called fractionator. A known such fractionatorutilizes a rotatable cylindrical drum, which is internally provided witha helical or spiral channel. This channel has a centrally locatedentrance and an exit located at the periphery of the drum. Thesuspension to be fractionated is introduced at the centre of the drum,and the drum is rotated so that the entrance end of the helical channelis filled like a scoop with suspension at each revolution. Between twoadjacent channel walls, thus, there will be a suspension plug movingoutwards towards the outer loop of the helix and the exit of thechannel. Thus, between the channel walls and the suspension plug therewill take place a relative movement. As is known from the science offlow this results in that larger particles will gather at the front endof the plug, whereas gradually smaller particles will gather graduallyfurther backwards in the plug, all as counted in its relative directionof movement. Seen in the direction of rotation of the drum, thus, thesmallest particles are located foremost and the largest last in theplug. Beneath the drum there are two or more collecting means, which arelocated in a row after and against each other in the rotationaldirection of the drum. When a suspension plug is situated in the lastturn of the helical channel, the entire plug leaves the exit of thechannel in a substantially coherent state when the exit moves over andpast the collecting means, the plug falling down towards the collectingmeans. Thus, in the first collecting means, as counted in the directionof rotation of the drum, the largest particles will be caught, while thesmallest will be caught in the last collecting means.

This known fractionator has an inherent drawback in that it operatesintermittently, since feeding of suspension and discharge of fractionsoccurs but once a revolution. Further, the fractioning distance, i.e.,the relative flow distance of the suspension, and, accordingly, thefractioning time is determined by the length of the helical channel.

In a not pre-published solution (SE-9303193-8) of the problemsassociated with this known fractionator, a fractionator is suggestedincluding a drum rotatable about a substantially horizontal axis andhaving axially spaced end walls. From an inlet centrally located in oneend wall of the drum, a flow channel extends forth and back between theend walls and radially outwards towards outlet means for the at leasttwo fractions in the other end of the drum. The flow channel is definedby substantially concentric cylindrical walls, of which every second inits one axial end is tightly connected to one end wall of the drum andevery second in its one axial end is tightly connected to the other endwall of the drum, so that flow can take place between the respectiveother ends of the cylindrical walls and the one and the other end wall,respectively, of the drum.

This fractionator has proven to comply well with all expectations asconcerns fractioning ability, but involves, of course, the drawback of arelatively complicated manufacture due to the cylindrical wallsalternatingly attached to the one and the other end wall. Thus, asimplification of the mechanical construction would be desirable. Inpractical tests with this fractionator it has surprisingly appeared thatfractioning is completed or at least sufficiently completed alreadyafter one passage between the end walls of the drum, i.e., from theinlet to the opposite end wall. Thus, further flow forth and back hasproven to be at least practically unnecessary and, to some extent, toreduce capacity as well.

Consequently, since it has surprisingly proven possible with reasonableaxial drum lengths to achieve a satisfactory fractioning merely bycausing a suspension to perform one axial passage along the length of adrum, there is proposed, according to the present invention, afractionator for fractioning a suspension in at least two fractions andincluding a drum rotatable about a substantially horizontal axis andhaving inlet for suspension and at least one outlet for fractions, whichis characterized in that the drum includes at least two walls curvedabout the axis and extending substantially parallel thereto in thelongitudinal direction thereof, said walls defining between them a fluidchannel substantially parallel to the axis, and in that the inlet isprovided in a first end of the drum and outlets for the at least twofractions in the other end of the drum.

The walls defining the flow channel may be coaxial cylinder walls oradjacent turns of a spirally curved wall.

The invention will be described hereinafter, reference being made to anexemplifying embodiment shown on the attached drawings, wherein:

FIG. 1 is an axial section through a fractionator according to thepresent invention having a drum with coaxial cylinder walls or aspirally curved wall,

FIG. 2 is an end view of a drum according to FIG. 1 seen from the outletside,

FIG. 3 is a view of a drum according to FIG. 1 having coaxial cylinderwalls, seen from the inlet side,

FIG. 4 is a view of a drum according to FIG. 1 having coaxial cylinderwalls, seen from the outlet side,

FIG. 5 is a view of a drum having spirally curved wall, seen from theoutlet side,

FIG. 6 is a part section at an enlarged scale through the upper leftportion of a drum according to FIG. 1, and

FIG. 7 shows the working principles of the fractionator according to thepresent invention.

The fractionator shown in FIGS. 1 and 2, which is adapted forfractioning a suspension in three fractions, includes a rotor in theshape of a fractioning drum 1 and a fractioning housing 2. The drum isrigidly supported by a substantially horizontal shaft 3 journalled inbearings 4, 5 axially outside the housing 2. The bearings are carried bya machine stand 6, 7. The shaft is driveable by means of a motor 8 and areduction gear 9 driveably connected to the motor and the shaft, suchthat the drum may be rotated at a relatively low rotational speed, forinstance 2,5-5, 5 rpm, typically 4 rpm.

The housing has parallel end walls 10 and 11, the wall 10 being providedwith an inlet 12 for suspension and the wall 11 with outlets 13, 14 and15 for three fractions.

The housing is liquid tight, at least up to the level of the shaft,where stuffing boxes 16, 17 seal between the shaft and the end walls 10and 11.

The housing has a cylindrical casing 18, and this and the end walls 10and 11 are divided along a horizontal diametrical plane through theshaft 3 in upper parts 18a and 10a, 11a, respectively, and lower parts18b and 10b, 11b, respectively, such that an upper part of the housingin the shape of a cover is liftable from a lower part constituting atrough for suspension. In the parting plane, at least the parts of thecylindrical casing, but preferably also the end walls, are provided withoutwardly directed flanges. In FIG. 2 is shown how the flanges of thecylindrical casing rest against an outer part 7' of the machine stand.In the end walls there are inspection hatches 19 and 20.

According to the present invention, the drum includes at least two wallscurved about the shaft and extending substantially parallel in thelongitudinal direction of the shaft, said walls defining between them aflow channel substantially parallel to the shaft. Between these wallssuspension may flow from the inlet end of the drum to its outlet end.For practical and not the least capacity reasons, however, thefractioning drum according to the present invention consists of severalparallel flow channels formed by either a plurality of concentriccylinder walls or a continuous wall curved in several spiral shapedturns.

In the example shown in FIG. 1, there are twenty-one such walls, thustwo by two defining twenty flow channels parallel to the shaft. For thesake of clarity, in FIG. 1 only the three radially inner walls and thefour radially outer walls have reference numerals, viz., 21, 22 and 23,and 24, 25, 26 and 27, respectively.

At the inlet side the radially inner channel wall 21 is tightlyconnected with (welded to) the peripheral region of a circular end plate28 constituting a hub for a number of radially directed spokes 29. Onthe outlet side, the channel wall 21 could be terminated without supportagainst the shaft, but it is preferred that it be also there tightlyconnected with (welded to) the periphery of a circular end plate 30. Theend plates 28 and 30 are tightly and non-rotatably connected with(welded to) the shaft 3. The channel wall 21, that is shown to have asubstantially greater material thickness than the remaining channelwalls, forms, together with the end plates 28 and 30, a stiffening meansfor the shaft 3 against torsion as well as bending.

The channel walls 22, 23 . . . 24, 25, 26 and 27 are all connected toand supported by merely the spokes 29 at one of their ends. This endconstitutes the inlet end of the flow channels extending betweenrespective pairs of adjacent channel walls, and the inlet of the drum isconstituted by the spaces 31 between adjacent spokes 29. At the oppositeend, the channel walls 22 . . . 27, being of equal length in the axialdirection, terminate entirely freely, so that the flow channels areentirely open at this end, their outlet end. Between the outlet end ofthe channels and the end wall 11 of the housing there is a littleclearance S (e.g., 0,5-1 mm).

In operation of the fractionator according to the present invention,suspension is introduced through the inlet 12 into the housing to aspace 32 between the end wall 10 and the inlet end of the drum, i.e.,the spokes 29 and the end plate 28. The suspension flow is adjusted sothat the level L at the inlet side of the drum, i.e., in the space 32,reaches up above the radially inner channel wall 21, e.g., just belowthe shaft 3. The suspension will now flow in through the inlet spaces 31between the spokes 29 and purely axially along the flow channels formedby pairs of adjacent channel walls 21-22, 22-23, . . . 25-26, 26-27, andout through the open ends of the flow channels to the outlet 13 of thefractionator.

Now, if the drum is rotated, an increase in the flow distance occurs. Ofcourse, such increase is depending on the rotational speed of the drum,since at a higher rotational speed two of the cylinder walls of thedrum, between and along which flow takes place, have time to rotate alonger distance during the time a certain volume of suspension ispresent therebetween, i.e., during the passage from the inlet end of thedrum to its outlet end. If, for instance, the rotational speed of thedrum at a certain axial flow is such that a certain volume of particleshas time to flow from the inlet end of the drum to its outlet end duringone revolution, the flow distance equals the diagonal of the rectangle,one side of which is the axial length of the channel wall and the otherside of which is the circumference of this channel wall (in case ofcoaxial cylinder walls) or the length of a spiral turn (in case of aspirally curved wall), respectively, i.e., longer than at one revolutionof the known fractionator having a helical channel and longer than atstationary fractionator drum according to the present invention. Thus,the path of flow describes a screw line, the pitch of which decreaseswith increasing rotational speed, i.e., that the liquid volume has timeto describe several revolutions relative to the channel wall during thepassage from the inlet end of the drum to its outlet end at increasingrotational speed. Thus, the fractioning distance is most considerablyincreased and, accordingly, the degree of separation between particlesof different sizes.

Since it has proven that relatively low rotational speeds and relativelyhigh flows result in sufficient fractioning, fractioning drums havingspirally curved wall can be used without any drawback, without anyconsiderable radial displacement of a flow channel taking place due tothe rotation of the drum.

Upon rotation of the drum in the direction indicated by arrows A inFIGS. 3 and 4, the surface of the suspension will be positionedapproximately as shown in FIG. 4, i.e., with increasing raising andlowering, respectively, towards the outer drum circumference due to therelative speed between the liquid and the rotating channel wallsincreasing towards that drum circumference.

Thus, in each flow channel defined by two adjacent channel walls, a partring of suspension having the cross sectional size of barely a halfcircle is moving from the inlet 32 towards the outlet end of the flowchannel. In such a cross section, that may be compared to the previouslymentioned suspension plug, a successive re-location of particles takesplace during the flow, so that at the outlet end the largest particlesare first in the cross section and the smallest last. Thus, the realfractioning is completed and the fractions could principly be taken careof by letting the flow fall freely down into two or more recipients fordifferent fractions placed in a row after each other under the outletend, e.g., as in the known fractionator.

It is preferred, however, to arrange the fraction outlets 13, 14 and 15mentioned. For the sake of clearness, these are drawn also in FIG. 4 andare arranged such that the outlet 15 is located first and the outlet 14last, as counted in the direction of rotation of the drum, and theoutlet 13 between the former ones. Counted in the relative direction offlow of the suspension plug, the order is reversed. Thus, the largestparticles are located in the region of the outlet 14, the medium-sizedin the region of the outlet 13 and the finest particles in the region ofthe outlet 15.

As indicated in FIG. 1 in connection with the outlet 13, and in FIG. 2in connection with all outlets, there are guide means 33, 34, 35, suchas metal plate shields or the like, to guide fractions from definedsectors of the drum towards the outlets 13, 14 and 15. In FIG. 2 thesesectors are located approximately in positions between 3 and 5 o'clock,between 5 and 7 o'clock, and between 7 and 9 o'clock. In order to adjustthe flow through the drum and particularly the level therein, at leastthe outlet located at the lowest level is provided with non-shownregulating means.

It is realized that a drum having coaxial cylinder walls and a drumhaving spirally curved continuous wall do not differ in axial section.The end view of a drum having spirally curved continuous wall 36 isshown in FIG. 6 only for the sake of completeness and is shown for thesake of clarity to have only six spiral turns.

In FIG. 7 is shown the operational principle for a fractionatoraccording to the present invention. In contrast to the knownfractionator, in which the flow direction and the fractioning directionwas one and the same, the flow here takes place in the axial directionof the drum, whereas the fractioning direction in a fractionatoraccording to the present invention is a resultant of an axial flowdirection and a relative flow direction depending on the rotation of thedrum, the fractions occupying different angular positions in thecircumferential direction of the drum as a result of rotation of thedrum. In practical use and in operation with rotating drum, therefore,the fractioning direction becomes substantially perpendicular to theaxial flow.

We claim:
 1. A fractionator for continuously fractioning differentfractions from a suspension, said fractionator comprising:a housinghaving inlet means for the suspension and outlet means for thefractions; a shaft; a drum positioned within said housing and beingrotatable on said shaft about a substantially horizontal axis; said drumhaving an open inlet end and an open outlet end and including at leasttwo fluid tight walls surrounding the axis and extending longitudinallysubstantially parallel thereto between said inlet end and said outletend to define between said ends a fluid channel extending substantiallyparallel to said axis from said inlet end to said outlet end andcontinuously extending in a circumferential direction of said drumthrough an arc of at least 360°; said outlet means comprising at leasttwo outlets for fractions, said outlets positioned at different angularpositions relative to said axis.
 2. A fractionator according to claim 1,wherein said walls are coaxial and cylindrical.
 3. A fractionatoraccording to claim 1, wherein said walls are formed by adjacent turns ofa spirally curved wall.
 4. A fractionator according to claim 1, whereinall said walls are supported from said shaft only at said inlet end. 5.A fractionator according to claim 1, wherein only an inner one of saidwalls is supported at both ends by said shaft and remaining walls aresupported only at said inlet end.
 6. A fractionator according to claim1, wherein said walls are supported at said inlet end by spokes radiallyextending from said shaft.
 7. A fractionator according to claim 5,wherein said housing includes an upper part and a lower part, whereinsaid lower part is a trough for suspension.
 8. A fractionator accordingto claim 1, wherein at least one of said at least two outlets isadjustable.
 9. A fractionator according to claim 1, wherein said wallshave an equal axial length.
 10. A fractionator according to claim 6,wherein said fluid channel has a channel outlet end, which is spacedwith little clearance from said at least two outlets.